Problems detecting hangup

Hello people, this is my first mail to this list. I’m new to asterisk
and trying to set up an IVR. So far my dialplan works nice connecting
with softphones, but I’m having problems to detect hangups on the analog
line.
Here are the details:

The problem as you already may know, is the busy/hangup tone generated
by the Panasonic PBX, connecting the asterisk boc directly to the pstn
works fine.
The Panasonic model is KX-TA616, as far as I understand, the manual[0]
says that it can detect Polarity reversion, but not generate it.
I recorded the hangup sound (using Record()), converted and opened in
audacity: it’s 260ms tone, 250ms
So I configured:
busydetect=yes
busycount=3
busypattern=260,250